The conventional technique of dithering is utilized to display many colors and grey scales on a display device having relatively few colors and grey scales without having to change the resolution of the display device. For example, through the use of the dithering technique, it is possible to display a 16-color image on a display device having only an 8-color palette. Similarly, by using dithering, it is possible to display an image formed from 16 grey scales on a binary display device in which each pixel can only be turned on or off. The underlying principle of dithering is to rely on a particular spatial distribution of illuminated pixels and non-illuminated pixels to reproduce the color and/or brightness of an original image on the display. When the original image is that of a natural scene such as scenery or portraits, the image displayed by this technique is very close to the original. However, when the original is a computer-generated image, the quality of the displayed image is very much distorted. Conventional dithering is explained in detail in a book entitled, "Fundamentals of Interactive Computer Graphics," by Foley and Van Damm (Addison-Wesley Publishing Co., Ltd., 1982) , at pp. 600-602. A 4.times.4 dither matrix is given therein as follows:
______________________________________ D.sub.00 D.sub.10 D.sub.20 D.sub.30 0 8 2 10 D.sup.(4) (i,j) = D.sub.01 D.sub.11 D.sub.21 D.sub.31 = 12 4 14 6 D.sub.02 D.sub.12 D.sub.22 D.sub.32 3 11 1 9 D.sub.03 D.sub.13 D.sub.23 D.sub.33 5 7 13 5 ______________________________________
The dither matrix D.sup.(4) maps into X-Y space as shown in FIG. 1(a). Specifically, D(X,Y)=D(i,j) for i=X mod 4 and j=Y mod 4. Conversion between a grey scale representation and a binary representation using conventional dithering may be illustrated by applying the dither matrix D.sup.(4) to the triangle shown in FIG. 1(b). For each point (X,Y) of the triangle, the following equation is applied: EQU if P.sub.old (x,y).gtoreq.D(x,y), EQU then P.sub.new (x,y)=1 EQU else P.sub.new (x,y)=0
where P.sub.old is the value of the pixel at location X,Y in the grey scale representation and P.sub.new is the value of the same pixel in the binary representation.
When the grey scale of the triangle in FIG. 1(b) is 5 (assuming a total of 16 grey scales with grey scale0=black and grey scale15=white), the results of applying this equation to the triangle are given below and are illustrated in FIG. 1(c).
______________________________________ P.sub.old (3, 1) = 5 &lt; D(3, 1) = 10 P.sub.new (3, 1) = 0 P.sub.old (2, 2) = 5 &gt; D(2, 2) = 1 P.sub.new (2, 2) = 1 P.sub.old (3, 2) = 5 &lt; D(3, 2) = 9 P.sub.new (3, 2) = 0 P.sub.old (4, 2) = 5 &gt; D(4, 2) = 3 P.sub.new (4, 2) = 1 P.sub.old (1, 3) = 5 &lt; D(1, 3) = 7 P.sub.new (1, 3) = 0 P.sub.old (2, 3) = 5 &lt; D(2, 3) = 13 P.sub.new (2, 3) = 0 P.sub.old (3, 3) = 5 = D(3, 3) = 5 P.sub.new (3, 3) = 1 P.sub.old (4, 3) = 5 &lt; D(4, 3) = 15 P.sub.new (4, 3) = 0 P.sub.old (5, 3) = 5 &lt; D(5, 3) = 7 P.sub.new (5, 3) = 0 ______________________________________
When the grey scale of the triangle in FIG. 1(b) is 11, the results are given below and are illustrated in FIG. 1(d).
______________________________________ P.sub.old (3, 1) = 11 &gt; D(3, 1) = 10 P.sub.new (3, 1) = 1 P.sub.old (2, 2) = 11 &gt; D(2, 2) = 1 P.sub.new (2, 2) = 1 P.sub.old (3, 2) = 11 &gt; D(3, 2) = 9 P.sub.new (3, 2) = 1 P.sub.old (4, 2) = 11 &gt; D(4, 2) = 3 P.sub.new (4, 2) = 1 P.sub.old (1, 3) = 11 &gt; D(1, 3) = 7 P.sub.new (1, 3) = 1 P.sub.old (2, 3) = 11 &lt; D(2, 3) = 13 P.sub.new (2, 3) = 0 P.sub.old (3, 3) = 11 &gt; D(3, 3) = 5 P.sub.new (3, 3) = 1 P.sub.old (4, 3) = 11 &lt; D(4, 3) = 15 P.sub.new (4, 3) = 0 P.sub.old (5, 3) = 11 &gt; D(5, 3) = 7 P.sub.new (5, 3) = 1 ______________________________________
As shown by this example, the conventional technique of dithering does not faithfully represent the original image. In both examples, the distribution of illuminated pixels on the binary display is not a faithful reproduction of the original grey scale image.
Other prior art dithering systems also do not perform entirely satisfactorily in representing an original image (see, e.g., Kubota, U.S. Pat. No. 4,930,022); Kimura, U.S. Pat. No. 4,914,524; Springer et al, U.S. Pat. No. 4,730,185; Larky et al, U.S. Pat. No., 4,956,638; Sautter et al., U.S. Pat. No. 4,377,821).
To overcome this disadvantage, the present invention has as its object to provide a method and an apparatus for a display device having relatively few colors or grey scales whereby the capability of the display device is enhanced so as to display faithfully more colors or grey scales than it would otherwise be able to display.